RFT3055LE Datasheet, PDF (7/10 Pages) Intersil Corporation – 2.0A, 60V, 0.150 Ohm, N-Channel, Logic Level, ESD Rated, Power MOSFET

RFT3055LE
Thermal Resistance vs. Mounting Pad
Area
The maximum rated junction temperature, T_J(MAX), and the
thermal resistance of the heat dissipating path determines
the maximum allowable device power dissipation, P_D(MAX),
in an application. Therefore the application’s ambient
temperature, T_A (^oC), and thermal impedance R_θJA (^oC/W)
must be reviewed to ensure that T_J(MAX) is never exceeded.
Equation 1 mathematically represents the relationship and
serves as the basis for establishing the rating of the part.
PD_(MAX) = ⁽---^T----^J---⁽---^M--_R---^A--_θ---^X-_J---_A⁾---–-----^T----^A----⁾-
(EQ. 1)
In using surface mount devices such as the SOT-223
package, the environment in which it is applied will have a
significant influence on the part’s current and maximum
power dissipation ratings. Precise determination of the
P_D(MAX) is complex and influenced by many factors:
1. Mounting pad area onto which the device is attached and
whether there is copper on one side or both sides of the
board.
2. The number of copper layers and the thickness of the
board.
3. The use of external heat sinks.
4. The use of thermal vias.
5. Air flow and board orientation.
6. For non steady state applications, the pulse width, the
duty cycle and the transient thermal response of the part,
the board and the environment they are in.
Intersil provides thermal information to assist the designer’s
preliminary application evaluation. Figure 22 defines the
R_θJA for the device as a function of the top copper
(component side) area. This is for a horizontally positioned
FR-4 board with 1oz copper after 1000 seconds of steady
state power with no air flow.This graph provides the
necessary information for calculation of the steady state
junction temperature or power dissipation. Pulse
applications can be evaluated using the Intersil device Spice
thermal model or manually utilizing the normalized maximum
transient thermal impedance curve.
200
R_θJA = 75.9 - 19.3 * ln(AREA)
147^oC/W - 0.026in²
150
128^oC/W - 0.068in²
110^oC/W - 0.171in²
100
50
0.01
0.1
1.0
AREA, TOP COPPER AREA (in²)
FIGURE 22. THERMAL RESISTANCE vs MOUNTING PAD
AREA
Displayed on the curve are the three R_θJA values listed in
the Electrical Specifications table. The three points were
chosen to depict the compromise between the copper board
area, the thermal resistance and ultimately the power
dissipation, P_D(MAX). Thermal resistances corresponding to
other component side copper areas can be obtained from
Figure 22 or by calculation using Equation 2. The area, in
square inches is the top copper area including the gate and
source pads.
R_θJA = 75.9 – 19.3 × ln (Area)
(EQ. 2)
8-149